1. Field of the Invention
This invention pertains generally to optical intensity modulators and more specifically to a design of a device that reduces the ohmic loss in the electrode structure of a traveling wave LiNbO.sub.3 intensity modulator without significant reduction of the modulator voltage-length product resulting in reduction of the high frequency drive voltage of the modulator.
2. Description of the Related Art
Traveling wave LiNbO.sub.3 intensity modulators are of great interest for analog radio frequency (RF) and microwave link applications, E-field sensor, and digital and analog communications. Of particular interest is the drive voltage of the modulator as this quantity determines link gain, sensor sensitivity, and drive power requirements for high-speed (40 GHz) analog and digital links. In velocity matched, traveling wave devices drive voltage is determined by the low frequency voltage-length product; velocity and impedance match; and electrical (Ohmic) losses in the traveling wave electrode structure.
High speed, broad bandwidth integrated optical modulators are made by constructing a traveling wave coplanar waveguide (CPW) electrode structure on the top surface of an optical waveguide modulator, typically made on a LiNbO.sub.3 substrate. In general these devices are Mach-Zehnder interferometers operated with a push-pull electrode structure, so that the fields of opposite polarity operate on each arm of the waveguide. These fields serve to change the index of the electro-optic LiNbO.sub.3, which in turn alters the phase of the light traveling in each waveguide, and thus allows operation of the interferometer. The optical phase or amplitude modulation results from an interaction between the optical wave in the optical waveguide and the microwave wave guided by the coplanar electrode structure. Bandwidth can be limited by optical-microwave phase mismatch (the two waves typically travel at different velocities, depending on the design of the device), by radio frequency (RF) or ohmic loss in the electroplated gold electrode structure, and by electrical coupling between the coplanar microwave mode and leaky substrate modes.
For a coplanar waveguide (CPW) traveling wave electrode structure on Z-cut LiNbO.sub.3, the electrodes are placed above the waveguides on the interferometer. The use of the etched regions in the LiNbO.sub.3 between and outside of the waveguides, resulting in "etched ridge" waveguides, has been shown to make it easier to achieve velocity matching for an impedance matched (near 50 Ohm) electrode structure. The geometry of the interferometer (separation of the waveguides) and the electrode structure (gap between the center and ground electrodes) are then interrelated in that the electrode gap essentially equals the waveguide separation. These quantities affect the modulator drive voltage differently, as follows, first, for a given voltage across the electrodes, increasing electrode separation decreases the electric field across the waveguides, as field.about.voltage/gap. This generally results in an increased voltage-length product and an increased drive voltage. Secondly, as the electrode gap increases the electrical losses in the CPW structure are known to decrease, resulting in lower losses along the line which would result in a decrease in high frequency drive voltage.
In the prior art, U.S. Pat. No. 5,416,859, Burns et al., issued May 16, 1995, a broadband electro-optic modulator is taught having a substrate of sufficiently small thickness so that coupling between the coplanar mode of the coplanar waveguide electrode structure and any one of the substrate modes of the substrate does not occur over a desired frequency bandwidth of operation, and has a coplanar electrode structure of sufficiently large thickness so that the second phase velocity of the electrical signal is substantially equal to the first phase velocity of the optical signal.
Typical values for the electrode and waveguide separation in use currently are 15-25 .mu.m. It is shown here that for Z-cut LiNbO.sub.3 values in the 40-80 .mu.m range can provide significantly reduced electrode losses without significant increase in voltage-length product. This will result in lower device drive voltage at higher frequencies.